Scalpels used in surgical procedures are extremely sharp and therefore somewhat dangerous to handle, particularly when being passed back and forth between the surgeon and an assistant. Unless close attention is used, which is not always possible under hectic circumstances, one or the other can sustain a cut to the hand or fingers which is not only painful but is otherwise highly undesirable. There has been a long-felt need for a scalpel that can be easily retracted before it is handed off to another person, and easily extended and locked for use in a surgical procedure. Various knives have been proposed which have retractable features. For example a Hughes U.S. Pat. No. 1,434,388 shows a knife blade having a small leaf spring on the inner end that engages notches on the edge of a channel in the handle to hold the blade in one of plurality of spaced positions. A button mounted on the spring is depressed to release the spring from the notch so that the blade can be shifted to another position. The spring is quite small and fragile, and thus apt to be broken or bent in use so that the blade is not restrained in any particular longitudinal position. The knife then becomes essentially useless because the blade can move freely in or out while cutting. A Costin U.S. Pat. No. 2,735,176 shows a spring-biased pin that extends through a hole in the blade. The pin is screwed to a button that has a boss which fits into enlarged holes at the opposed ends of a longitudinal slot in the handle. The button must be pulled outward against the bias of the spring to disengage the boss so that the blade can be extended or retracted until the boss drops back into a hole under the influence of the spring. This knife is awkward to operate because it must be held in one hand while the fingers or fingernails on the other hand are used to pull the small button out and compress the spring to disengage the boss. Particularly where the user is wearing surgical gloves that have become somewhat slippery, efforts to manipulate the button can be exasperating.
A general object of the present invention is to provide an new and improved retractable blade cutting instrument that obviates the disadvantages noted above.